This invention relates to apparatus for providing from two quadrature input signals, which each is a function of a variable, a signal proportional to the variable. The invention also relates to an open-loop fiber optic gyroscope system comprising such apparatus for providing a signal proportional to the so-called Sagnac phase shift.
An open-loop fiber optic gyroscope with electronic closed-loop "phase shift nulling"-type electronic signal processing to perform analogue phase tracking or extraction is disclosed in a letter by A. D. Kersey and R. P. Moeller published in ELECTRONIC LETTERS Vol 26, No. 16 of Aug. 2, 1990 on page 1251. In the system disclosed, two quadrature signals derived from the open-loop fiber optic gyroscope and which are functions of the Sagnac phase shift, are fed to a trigonometric transformation circuit. An input of a first order dynamic control circuit comprising an integrator is connected to the trigonometric transformation circuit. The system output, which is proportional to the Sagnac phase shift to be extracted, is provided at an output of the dynamic control circuit. A feedback loop is provided between the dynamic control circuit and the trigonometric transformation circuit.
The aforementioned system has been found to have unsatisfactory ramp and step responses. Furthermore, because the trigonometric transformation circuit comprises commercially available analogue trigonometric function generators with a limited operational signal range (typically about .+-.540 degrees), the maximum detectable rotation rate of the system is limited. For example, for a system with a gyroscope with a scale factor of .phi./.OMEGA.=10 s, the maximum detectable rotation rate is about .+-.54.degree./s.